The outer neomembrane of chronic subdural hematoma.

Thomas Willis recognized the entity of the chronic subdural hematoma (CSH) more than 300 years ago. Modern observations of the ultrastructure of normal meninges and the CSH outer neomembrane, combined with techniques of radioactive red blood cell labeling and chemical analysis of CSH fluid, have provided insight into the pathophysiology of this entity. The outer membrane evolves as a reaction to cleavage of the dural border cell layer of the meninges.